I've read this discussion from the start and it's intrigued me. I'm one of the guys behind the Super Soaker Central website. I actually have made spud guns before (for the record) but it's not something I do on a daily basis.

I was thinking of a constant pressure against the water directly by the piston and springs(rubber tubing).

In that manner, you wouldn't get constant pressure. The force rubber tubing applies increases with displacement so the pressure will decrease as water is shot out.

The reason that filling rubber tubing with water gives approximately constant pressure is through the simple equation F / A = P. The total applied force on the water approximately in the same proportion that the surface area increases in latex tubing. Consequently the pressure is relatively constant.

The two easiest ways to get constant pressure are through regulated gas pressure and latex tubing. I've examined other methods but they often are complicated and expensive.

LOL, When I saw all the purple primer and the one with the styrofoam and tape barrel support, I doubt those designers are also spudders.

Now I'm a little cleaner and don't use styrofoam. The styrofoam was just what was lying around my house. Wood works a lot better.

If you're interested in building something like Supercannon II I'd suggest using a 4 inch cap on one end (with threads tapped for 1/4 inch fittings to fill it with air) and not using 4 inch bell reducers on the other end. While I haven't died yet from using DWV bell reducers and I doubt anything bad would happen from it I'd recommend using something rated for safety. Pressure rated 4 inch to 2 inch reducers are not made in schedule 40 (believe me, I've gone directly to pipe manufacturers) so your cheapest option would be a coupling and a bushing. You can get the right pressure rated reducer in schedule 80 but you'll have to pay over $100 for it so it's not worthwhile unless you want to reduce weight slightly.

The other thing I'd recommend would be adding a larger air chamber like an over-under cannon with a pressure regulator. Then you can use the entire pipe for water and even more impressive blasts. You also could attach it to an air compressor to get the same effect.

Watch out for recoil when using one of these. They'll easily catch you off guard. There's much more recoil than you'd imagine because of the duration of the shot.

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Hope you weren't offended by my comments, it's more a spudder thing and difference between the two.

There seems to obviously be much in common between the concepts of spudding and super soaking.

I agree with all the information you have given and find it interesting.
Most of the science and concepts are the same, so there's a lot to learn/study for both fields.

Well, it's known that pipe failure involving water is much safer than pipe failure involving compressed gas.
I know it's not good to design around that rather than be certain by using rated (and not damaged by UV or time) components.

I was also thinking about how great a design could be using regulated pressure (much info on that here)

With the constant pressure from latex bands, true, the further they're pulled the more force they apply, however at a further distance they're pushing more water , which compensates some (depending on nozzle length)
Also if a pulley is used similar to a compound bow, the bands are set to never be fully "at rest" yet still compact.

Hope you weren't offended by my comments, it's more a spudder thing and difference between the two.

No problem.

Well, it's known that pipe failure involving water is much safer than pipe failure involving compressed gas.

You have to watch out for different things. In water guns the main thing to watch out for in my experience are parts that will turn into water rockets. Typically this only happens when hose clamps are involved. High torque hose clamps, threaded hoses, and reasonable pressures are the solution.

With the constant pressure from latex bands, true, the further they're pulled the more force they apply, however at a further distance they're pushing more water , which compensates some (depending on nozzle length)

In my experience any difference from the increased weight of the water is negligible. If it made a difference than air and spring pressure would be more constant than they are. I don't know what you mean by nozzle length though. Typically the length of the nozzle makes little difference in performance.

With that being said, not having constant pressure isn't necessarily bad. It's just not ideal.

Also if a pulley is used similar to a compound bow, the bands are set to never be fully "at rest" yet still compact.

I'm not quite sure what you mean. Could you make a drawing? This idea sounds useful but I'm not sure if I have the right image in my mind.